1. Field of the Invention
The present invention relates to an ultrasonic (or supersonic) driving mechanism, such as an ultrasonic actuator and an ultrasonic motor, utilizing an electrical-mechanical energy converter such as a piezoelectric vibration element.
2. Description of the Related Arts
With reference to FIG. 1, an example of a conventional ultrasonic actuator of truss type, as an ultrasonic driving mechanism, is described below. As shown in FIG. 1, the ultrasonic actuator has a base 1 to which a pair of elongate piezoelectric vibrators 2 and 3 are fixed, for example, by means of adhesive. Each of the elongate piezoelectric vibrators 2 and 3 is composed of a plurality of piezoelectric plates which are laminated, or layered, one over another. The pair of elongate piezoelectric vibrators 2 and 3 are fixed to the base 1 with a predetermined angle between the pair of vibrators 2 and 3. The apexes of the vibrators 2 and 3 are fixed to each other via a resultant part 4 made of a rigid body, for example, by means of adhesive.
When an unshown control circuit applies a voltage having a predetermined phase difference to the piezoelectric vibrators 2 and 3, they expand and contract (i.e. vibrate) respectively with the phase difference therebetween. Thereby, the resultant part 4 makes an elliptical motion. A pressurizing portion, constituted by a spring 7, presses the resultant part 4 against a surface of a to-be-driven member (i.e. a member to be driven) 8. Thereby, the to-be-driven member 8 is driven to rotate about a rotational center C at a predetermined circumferential speed of Vr.
By the way, the to-be-driven member 8 which is driven by the ultrasonic actuator, is not limited to a rotor as shown in the figure. Namely, the to-be-driven member 8 can be a member which moves linearly.
FIG. 2 illustrates the elliptical motion of the resultant part 4 shown in FIG. 1. A point “A” in the elliptical locus shows a point at which the resultant part 4 starts to contact the to-be-driven member 8. A point “B” shows another point at which the resultant part 4 is detached from the to-be-driven member 8. A point “C” shows the intermediate point between the points “A” and “B”. Because the resultant part 4 is made of the rigid body, it draws the elliptical locus shown in FIG. 2, irrespective of whether the resultant part 4 contacts with the to-be-driven member 8. That is, components VAX, VBX, and VC of speed at the points A, B, and C in the X-axis direction are kept at a constant value, respectively, irrespective of whether the resultant part 4 contacts with the to-be-driven member 8. As a result, there occurs a slip between the resultant part 4 and the to-be-driven member 8, which causes an energy loss.
On the other hand, Japanese Laid-Open Patent Publication No. 55-125052, discloses an actuator in which vibrations are transmitted, or imparted, to its to-be-driven member via an elastically deformable member which is in the form of a projection. With the arrangement, it is possible to prevent a slip between the elastically deformable member and the to-be-driven member. But the deformable member extends with a state in which the deformable member inclines at a certain angle with respect to the to-be-driven member. Thus, with the arrangement, the to-be-driven member can be driven in only one direction. If a pair of actuators are arranged to the to-be-driven member, it is not possible to drive the to-be-driven member properly, because there exists such a problem that when the to-be-driven member tries to be driven by one actuator, the elastically deformable member in the form of the projection of the other actuator interferes with that of the above one actuator.